The problem is resolved, I think… It seems to have been related to my use, or rather misuse, of the media library. I had a number of attempts and false starts with setting up a gallery, and each time I’d just drag the photos from my file manager into the gallery. This resulted in multiple copies of the same file in the media library. I cleaned out the library and now bring files into the gallery from the media library instead of dragging them from my file manager, at least if they’re already in the library.